(1) Field of the Invention
The present invention relates to a deflection yoke, and particularly to a deflection yoke which is capable of damping the magnetic fields leaking to the front of the screen.
(2) Description of the Prior Art
Generally, a deflection yoke includes: a pair of semi-trumpet shaped pieces made of synthetic resin; a horizontal deflection coil disposed within the semi-trumpet shaped pieces for providing a horizontal deflection magnetic field for deflecting electron beams; and a vertical deflection coil disposed to the outside of the semi-trumpet shaped pieces to provide a vertical deflection magnetic field to the electron beams.
The deflection yoke constituted as described above is installed on the neck portion of cathode ray tube. If horizontal and vertical saw teeth-shaped wave currents are supplied to the deflection yoke, the horizontal and vertical coils form deflection magnetic fields, and these magnetic fields deflect the electron beams from an electron gun up and down and leftwardly and rightwardly, thereby distributing the electron beams to the raster of the screen.
However, of the coils for deflecting the electron beams, the horizontal coil produces superfluous magnetic fields to the front of the screen. Such leaking magnetic fields not only give adverse effects to the electric or electronic apparatuses disposed around the screen, but also inflict harmful effects to the human body.
Accordingly, coming recently, a stern standard is introduced against the leaking magnetic fields which are produced from the above described deflection yoke.
One conventional way of protecting against leaking magnetic fields is to cover the cathode ray tube and deflection yoke with a conical metal shield during installation in the picture display device. However, such a protecting shield is effective in shielding the picture displayer from external magnetic fields rather than keeping the magnetic fields of the deflection yoke from being leaked to the front of the screen.
Meanwhile, Japanese Patent Application Laying-opening No. Sho-62-100935 discloses a displaying tube which includes: a rear portion for accommodating at least one of an electron beam generating unit, and a front portion consisting of a phosphorescent screen, in order to inhibit the leaking magnetic fields without using the protecting shield. Further, there is provided an electron deflection unit consisting of a line deflection coil and a field deflection coil for producing a magnetic field having a dipole component if energized. This electron deflection unit is capable of deflecting the electron beams across the displaying screen, and is disposed at the periphery of the displaying tube. This displaying tube is provided with an inhibiting coil, and the posture of this inhibiting coil is adjusted in such a manner that at least a part of the dipole magnetic field detectable at a distance from the screen is kept to below the standard, thereby keeping a leaking magnetic field from being produced (Refer to FIGS. 1a and 1b).
This device is based on the understanding that, if a leaking magnetic field is to be inhibited at a distance of 3 m from the magnetic field source, the dipole component of the magnetic field has only to be compensated. Therefore, this device provides a method of installing an inhibiting coil for inhibiting the dipole component of the magnetic field by means of a line deflection coil. That is, as shown in FIG. 1a, an impediment control coil 13 is installed above a picture displayer 11 and within a cabinet 12 which houses the picture displayer 11. Further, as shown in FIG. 1b, a pair of degaussing coils 15a, 15b are installed around the funnel portion of the tube.
However, this method has the problem of cost increase, and with regard to the damping of the impeding magneting field, no satisfactory result has been achieved.
Further, Japanese Patent Laying-opening No. Sho-64-45046 discloses a cathode ray tube in which a ferrite ring is installed between the screen and the deflection yoke in order to reduce the magnetic fields leaking to the front of the screen from the deflection yoke. According to this method, however, if the horizontal deflection frequency has a low frequency, a high permeability ferrite ring has to be used, while, if it is a high frequency, a low permeability ferrite ring has to be used, this being a problematic feature.
Further, Japanese Patent Laying-opening No. Sho-62-223592 discloses a method in which a current having a time function corresponding to the time function for the current supplied to the horizontal deflection coil (producing a stray magnetic field) is supplied to a stray magnetic field reducing current line, so that the magnetic field produced by the current line should be able to neutralize the stray magnetic field related to the stray magnetic field generation region. The stray magnetic field reducing current lines 22 according to the above described method as shown in FIGS. 2a to 2d are disposed on the upper and lower edges of the front face of a picture displayer face plate 21 in the horizontal form, and are also disposed on the left and right edges of it in the vertical form, while they are also connected to the deflection yoke in parallel or in series.
According to this method, however, it is a complicated process to install the current lines near the face plate of the cathode ray tube, and the manufacturing cost is increased, while its effect is not significant.